The MIMO system is a communication system using a plurality of transmission antennas and a plurality of reception antennas for data communication. An access point to which a user terminal is connected can conduct communication with one user terminal or more at an arbitrary required point in time over a downlink and an uplink. The downlink (i.e., a forward link) is a communication link originating from the access point to the user terminal, whilst the uplink (i.e., a backward link) is a communication link originating from the user terminal to the access point.
The access point is usually a wireless communication device realized by a stationary base station that establishes communication with the user terminal and is often called a base station or in another technical term. The user terminal is a stationary or mobile wireless communication device and often called a base station, a wireless device, a mobile station, user equipment, or in another technical term. In the following descriptions, a base station (BS) is used for the access point, and user equipment (UE) is used for the user terminal.
A closed-loop MIMO system usually transmits channel status information from a receiver to a transmitter. Specifically, when performing precoding or beamforming operation, the closed-loop MIMO system uses channel status information fed back from the receiver to the transmitter in the communication system, thereby implementing channel optimization. Precoding is a technique of, when a plurality of antennas in a MIMO system make a transmission, forming a beam appropriate for circumstances of a propagation path by means of transmitting weighted data from the respective antennas, thereby making the transmission. In order to cause the transmission to reflect circumstances of observation of a received signal achieved at a receiving point (i.e., propagation path conditions), a feedback signal including beam information is transmitted at this time from the receiver to the transmitter, and the transmitter controls the beam by use of the feedback signal (see; for instance, Patent literature 1). Precoding has been under debate in relation to LTE (Long Term Evolution) of a next-generation system whose standardization activity has been performed on a 3GPP (3rd Generation Partnership Project) that is an international standardization organization of a portable phone.
In relation to transmission of the MIMO system, when a higher-order MIMO system uses a maximum of eight transmission antennas for a downlink channel or when spatial limits are imposed on setting of an antenna for an uplink channel in a user terminal of a MIMO system, utilization of a cross-polarized antenna structure is effective. If cross-polarized antennas assigned different polarized waves are used respectively for a transmitter and a receiver, an imbalance will occur in gains (or power) of and correlation among elements of a channel matrix. As a consequence, the elements of the channel matrix exhibit more complex behaviors. However, in order to lessen complexity of implementation and retain one code book intended for all applications (various antenna structures), the current LTE includes no code book specifically designed for the cross-polarized antenna structure. Since the cross-polarized antenna structure exhibits distinguished usefulness, distinguished performance advantages will be yielded as a result of addition of a code book of this type at the time of development of the LTE to a next advanced LTE.
A code book using a section diagonal matrix is conceivable as being intended for precoding operation of a MIMO system using a cross-polarized antenna structure (hereinafter referred to as a “cross-polarized MIMO system”). A premise for ideal XPD (Cross Polarization Discrimination) is applied for the code book of this type. In this case, the channel matrix can be approximated by a section diagonal matrix. In general, however, ideal XPD cannot always be expected. A precoding matrix using a code book of this type cannot be matched to a structure of a channel matrix at this time. Therefore, when conditions for ideal XPD are not satisfied, precoding performance is deteriorated. As mentioned above, a necessity for making the precoding technique for the cross-polarized MIMO system efficient from both a transmission performance viewpoint and a signaling viewpoint exists in this field.